Treatment of textile wastewater by homogeneous and heterogeneous Fenton oxidation processes

Desalination 281 (2011) 438–445 Contents lists available at SciVerse ScienceDirect Desalination journal homepage: www.elsevier.com/locate/desal Treatment of textile wastewater by homogeneous and heterogeneous Fenton oxidation processes S. Karthikeyan, A. Titus, A. Gnanamani, A.B. Mandal, G. Sekaran ⁎ Environmental Technology Division, Council of Scientific & Industrial Research (CSIR)-Central Leather Research Institute (CLRI), Adyar, Chennai, 600 020, India a r t i c l e i n f o Article history: Received 14 May 2011 Received in revised form 28 July 2011 Accepted 13 August 2011 Available online 14 September 2011 Keywords: Heterogeneous Fenton oxidation Textile wastewater Mesoporous activated carbon Homogeneous Fenton oxidation a b s t r a c t In the present investigation an attempt was made to degrade organic pollutants in the textile effluent by homogeneous and heterogeneous Fenton systems. Experiments were carried out under the batch as well as under the continuous operating conditions. The effect of time, pH, H2O2 concentration, FeSO4.7H2O concentration and the mass of mesoporous activated carbon on the degradation of organics in the wastewater were critically examined. The kinetic constants and the thermodynamic parameters for the oxidation of organics in wastewater were determined. The quantitative removal of COD, BOD and TOC from the wastewater was evaluated. The degradation of organics in textile wastewater was confirmed through FT-IR, UV–Visible spectroscopy, and cyclic voltammetry. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Textile Industry employs a wide spectrum of chemicals viz. enzymes, detergents, dyes, acids, sodium salts etc. for the conversion of natural fibers into textile fibers. The conventional treatment systems that are used in the effluent treatment plants or in the common effluent treatment plants, include primary clarifier, secondary biological aerobic system, secondary clarifier, sand filter and activated carbon filter [1]. The wastewater discharged from the textile industry seldom meets the discharging standards prescribed by the pollution control boards in India. This is due to the presence of high COD to BOD ratio, unspent dyes and total dissolved solids. The poor biodegradability of wastewater is due to the presence of high concentration of azo dyes. The major pollutants identified in the textile wastewater are high pH, color, nutrients (nitrogen and phosphorus), inorganic salts and refractory organics [2,3]. The azo and other chromophoretic groups in the dye matrix render geno toxicity to the biodiversities in the environment. Advanced Oxidation Processes (AOPs) have been suggested for the partial or complete removal of pollutants in the wastewater or their transformation into less toxic and more biodegradable products. Furthermore, the partial decomposition of non-biodegradable organic pollutants can lead to biodegradable intermediates. There are indications of partial decomposition, elimination of chromophoretic groups, increase in the solubility index of the compounds and reduction in the aromaticity which lead to an improvement in the biodegradability of the textile wastewater by AOPs. ⁎ Corresponding author at: Environmental Technology Division, Central Leather Research Institute, Adyar, Chennai, 600 020, India. Tel.: + 91 44 24452491; fax: + 91 44 24912150. E-mail address: ganesansekaran@gmail.com (G. Sekaran). 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.08.019 Fenton oxidation, one of the AOP's was studied extensively by many researchers for the treatment of wastewater containing refractory/xenobiotic organic compounds [4,5]. The sequential steps that take place in the process of Fenton oxidation of organics in wastewater are shown below. 2þ H2 O2 þ Fe →Fe 3þ þ OH− þ OH• OH• þ RH→CO2 þ H2 O k ¼ 70 M−1 s−1 ð1Þ k ¼ 109 –1010 M−1 s−1 ð2Þ R• þ Fe3þ →Rþ þ Fe2þ ð3Þ H2 O2 þ FeðOHÞ3 →H2 O þ O● þ FeðOHÞ3 ð4Þ Eq. (1) suggests that there is a wide scope for the precipitation of ferric ion with hydroxide ions as ferric hydroxide. The ferrous ion is excessively used in order to sustain the Fenton oxidation reaction. This demands a post treatment for the Fenton oxidized wastewater for the decontamination of heavy metal pollution. Moreover, the ferric hydroxide sludge produced is the source for the decomposition of hydrogen peroxide. Thus, the generation of sludge and huge consumption of chemicals were the identified disadvantages of homogeneous Fenton oxidation of organics in wastewater. This can be overcome by the following two methods (i) providing a sink to abstract an electron from hydroxyl ion to generate hydroxyl radical [6] and (ii) providing a second matrix phase to prevent the ferric ion from combining with the hydroxyl ion. Mesoporous Activated Carbon (MAC) has been shown to possess the above two characteristics. The active sites of MAC are made available for the adsorption of organic compounds according to the energy S. Karthikeyan et al. / Desalination 281 (2011) 438–445 439 possessed by them [7]. The organic compounds are oxidized by hydroxyl radicals into their end products. Considering the favorable characteristics of MAC many researchers have applied it as heterogeneous catalyst for the oxidation of organics in wastewater [8–10]. However, it has been observed that the application of MAC for the Fenton oxidation of organics in the textile wastewater is limited. Thus, the focal theme of the present investigation was to demonstrate the performance of the Homogeneous followed by Heterogeneous Fenton oxidation processes (using MAC as heterogeneous catalyst) for the removal of dissolved organics in the wastewater at low residence time and with the low consumption of Fenton's reagent under batch and continuous mode operations [11]. 0.6 kg/cm 2 was applied at ambient temperature and pressure maintained for homogeneous and heterogeneous Fenton oxidation processes. The COD measured in the wastewater samples after Fenton oxidation of organics was corrected in accordance with the equation shown below to prevent the interference of H2O2 in COD analysis 2. Materials and methods 2.5. Integrated homogeneous and heterogeneous catalytic treatment of organics in textile wastewater under continuous mode CODA ¼ CODM −RP  0:25 ð5Þ where CODA is the actual COD in the sample; CODM is the measured COD and RP is the residual hydrogen peroxide in the wastewater sample after Fenton oxidation. 2.1. Preparation of electron rich activated carbon matrix Rice husk, a precursor material obtained from the agro industry was used for the preparation of MAC in sequential steps such as carbonization at 400 °C and activation using phosphoric acid at 700 °C, 800 °C and 900 °C. The activated carbon samples were washed with distilled water until the wash water showed negative test for phosphate. The washed mesoporous activated carbon was dried at 110 °C to get the finished product and they were labeled as MAC700, MAC800, and MAC900 respectively. 2.2. Characterization of MAC samples MAC700, MAC800, and MAC900 samples have been characterized for surface area, pore volume and pore size distribution using an automatic adsorption instrument (Quantachrome Corp. Nova-1000 gas sorption analyzer). The free electron density of MAC was determined by electron spin resonance spectroscopy (Bruker IFS spectrophotometer ESR). The elemental composition (Carbon, Hydrogen and Nitrogen content) of the MAC samples was determined using CHNS 1108 model Carlo-Erba analyzer. A Philips X' pert diffractometer was used to determine the crystallites present in the MAC800 sample by X-ray diffraction technique (XRD) and a Perkin-Elmer infrared spectrophotometer was used for the investigation of the surface functional groups in the MAC samples. The surface morphology of MAC was determined using Leo–Jeol scanning electron microscope and the Energy gap value of MAC was determined by using Diffuse Reflectance Spectroscopy. 2.3. Collection and pre-treatment of wastewater The wastewater was collected from the overflow of a primary clarifier in an effluent treatment plant of a textile industry in Tamil Nadu, India. The wastewater was screened to remove the floating solids and was sand filtered to remove the suspended solids. 2.4. Fenton oxidation of organics in textile wastewater under batch mode The effect of pH (2.5, 3.5, 4.5, 5.5, 7.0), the effect of concentration of hydrogen peroxide (3, 6, 9, 12 and 15 mM/L) and the effect of concentration of FeSO4.7H2O (0.4, 0.8, 1, 1.4 to 1.8 mM/L) and effect of temperature (303, 313, 323, 333 K) were carried out to determine the optimum conditions for the oxidation of organics in the textile wastewater. The process consists of adjusting the pH of the sand filtered textile wastewater to the desired pH level using sulfuric acid (36N, sp. gr. 1.81, 98% purity). The homogeneous Fenton oxidation was carried out by dosing the textile wastewater with H2O2 and FeSO4.7H2O. Aliquots of samples were withdrawn every 1 h and analyzed for pH, ORP, BOD, COD and TOC according to the procedure reported in the standard methods for analysis of water and wastewater. The heterogeneous Fenton oxidation was carried out by dosing the wastewater with H2O2 (30% w/v) and FeSO4.7H2O followed by the addition of MAC800 (10 g/L). Air at a flow rate of 1.2 L/h and at a pressure A PVC reactor of height 50 cm and diameter of 6 cm was fabricated. The volume of reactor was 1 L with the working volume of 400 mL. The bottom of the reactor was filled with quartz stones covering a height of 8 cm and gravel above it for about 2 cm. The reactor was then filled with 150 g of the MAC and provision was made to distribute air in the MAC bed to facilitate oxygen transfer for oxidation as shown in Fig. 1. The outlet of the homogeneous reactor was the inlet to the heterogeneous reactor in the integrated catalytic treatment process. For simplification purposes the integrated catalytic treatment process is defined as heterogeneous process throughout this manuscript. The pH of the textile wastewater was adjusted to 3.5 using H2SO4 (36N, specific gravity of 1.81). 2 mM/L of H2O2 (30% w/v) and FeSO4.7H2O (0.1 g/L) were added to the wastewater and then applied to the reactor through sample distribution system provided at the top of the reactor in the down flow direction. The flow rate was maintained in such a way that the hydraulic retention time provided was 1 h. The aliquots of sample were collected and analyzed for pH, ORP, BOD COD and TOC. 2.6. Instrumental analysis The raw and the Fenton oxidized samples were scanned using UV–Visible spectrophotometer (Varian, Cary 100 Conc.). The FT-IR spectra of the MAC sample, raw wastewater sample and the treated wastewater sample were recorded using Perkin-Elmer infrared spectrophotometer for the identification of the functional groups. The dried samples were mixed with spectroscopic grade potassium bromide and made in the form of pellets at a pressure of about 1 MPa of dimensions; diameter, 10 mm and thickness, 1 mm. The samples were scanned in the spectral range of 4000–400 cm − 1. The Cyclic Voltagram was recorded for the untreated and treated wastewater samples at potential in the range + 10 V to − 10 V and current in the range + 250 mA to − 250 mA with a cyclic voltammeter (CHI600D type). The CV was recorded with Ag/AgCl as reference electrode and Pt wire as counter electrode. The working electrode was Pt and the scan rate was 0.1 V/s. 2.7. Physico-chemical analysis of the wastewater The primary treated, sand filtered and the Fenton oxidized wastewater samples were analyzed for pH, BOD5 (Biochemical Oxygen Demand), COD (Chemical Oxygen Demand), TOC (Total Organic Carbon) and TDS (Total Dissolved Solids), according to the methods summarized in the standard methods for the analysis of wastewater [12]. 3. Results and discussion 3.1. Characterization of MAC The characteristics of MAC samples are presented in Table 1. The maximum reflectance at a wavelength corresponding to 800 nm 440 S. Karthikeyan et al. / Desalination 281 (2011) 438–445 Pressure release Pressure release H2O2 B A FeSO4 Raw Effluent pH Correction Air Blower Treated Effluent Air Blower Fig. 1. Schematic flow diagram for integrated homogenous and heterogeneous Fenton oxidation of textile wastewater. A: — Homogeneous Fenton oxidation reactor B: — Heterogeneous Fenton oxidation reactor. 3.2. Collection and characterization of textile waste water The wastewater samples were collected from the overflow of a primary clarifier from an ETP of a textile industry in Tiruppur, Tamil Nadu. The wastewater sample was sand filtered to remove the suspended solids. The wastewater was characterized for pH, ORP, BOD, COD, and TOC, Total solids, Chloride, TKN, VSS and hardness. 3.3. Fenton oxidation of textile wastewater 3.3.1. Effect of time The percentage removal of COD, BOD and TOC varied with time as shown in Fig. 2. The percentage removal of COD increased linearly up to 4 h and was followed by a non-linear increase up to 50% in 6 h. The initial linear increase in COD reduction may be attributed to the chemical oxidation of the dissolved organics in the wastewater with OH•. In Heterogeneous Fenton oxidation of organics, the percentage removal of COD with time also followed an initial linear increase and then followed by a non linear increase. The percentage reduction in COD increased slowly to 90% in the heterogeneous Fenton oxidation of the textile wastewater in about 4 h. The homogeneous Fenton oxidation removed TOC by 64.1% and BOD by 61% and heterogeneous Fenton oxidation removed TOC by 90% and BOD by 88%. 3.3.2. Kinetic study Fig. 3. shows that the removal of COD from the textile wastewater varied as a function of time in homogeneous Fenton oxidation process. The reaction kinetics fitted well for pseudo first-order reaction with regression coefficient 0.98. The kinetic model suggested that the rate of reaction was limited only by the concentration of hydroxyl radical. The rate constants for the reactions at 303, 313, 323 and 333 K were found to be 0.0016, 0.0023, 0.0032 and 0.004 min − 1. However, the heterogeneous Fenton oxidation process followed second order rate kinetic model (Fig. 4) with a regression coefficient of % Removal of COD,BOD and TOC correlates with energy gap (Eg) of 1.55 eV in MAC800, which falls in the extrinsic semiconductor range. MAC800 was selected as heterogeneous catalyst for the oxidation of dissolved organics in textile wastewater for its optimum characteristics of surface area, pore volume, pore diameter, C, H, N, ash content and free electron density respectively as 379 m 2/g, 39.36 Å, 41.5%, 2.85%, 0.75%, 41.6% and 1.60 × 10 22 spins/g. 100 80 60 40 % COD Removal With MAC % COD Removal Without MAC % TOC Removal With MAC % TOC Removal Without MAC % BOD Removal With MAC % BOD Removal Without MAC 20 0 0 50 100 150 200 250 300 350 400 Time, min Fig. 2. Percentage removal of COD, TOC and BOD as a function of time in homogeneous Fenton and heterogeneous Fenton oxidation of textile wastewater. Table 1 Characteristics of mesoporous activated carbon. S. No. 1 2 3 4 5 6 7 8 Parameters Surface area (m2/g) Average pore diameter (Ǻ) Carbon (%) Hydrogen (%) Nitrogen (%) Free electron density (spins/g) Energy gap (eV) Ash content (%) Values MAC700 MAC800 MAC900 345 38.82 42.56 3.14 0.82 8.51 × 1020 1.35 41.24 379 39.36 41.58 2.85 0.75 16.05 × 1021 1.55 41.60 439 35.28 37.96 2.40 0.50 15.98 × 1021 1.52 45.68 Fig. 3. Determination of rate constant and order of homogeneous Fenton oxidation of textile wastewater. 441 S. Karthikeyan et al. / Desalination 281 (2011) 438–445 -lnk vs 1/ T y = 4592.1x - 8.3402 8 2 R = 0.9693 7 6 y = 2312.5x - 3.7497 -lnk 5 2 R = 0.9936 4 3 2 with MAC 1 Fig. 4. Determination of rate and order of the reaction for heterogeneous Fenton oxidation of wastewater. 0.98. The mathematical expression for validating the heterogeneous Fenton oxidation process was 1=½AŠt ¼ 1=½AŠ0 þ kt ð6Þ This suggests that the concentration of hydroxyl radical and the number of active sites present in MAC limited the rate of heterogeneous Fenton oxidation of organics in textile wastewater. The rate constants for the reactions at temperatures 303, 313, 323, and 333 K were found to be 0.0202, 0.02695, 0.03368 and 0.04042 L/mg.min respectively for heterogeneous Fenton oxidation of textile wastewater [13–16]. The increase in rate constant of Fenton oxidation with temperature suggests that mobility of reactants to the heterogeneous surface from the bulk medium and the converted products from the surface to the bulk medium are more favored by the applied thermal energy. Thus, the active sites remain unanchored by the organic molecules leading to sustain Fenton oxidation of textile wastewater. The rate constants for oxidation of textile wastewater determined at different temperatures were used to calculate the activation energy of reaction by the following expression [17]: ln k ¼ − Ea þ lnA RT ð7Þ where, Ea is the Arrhenius activation energy for the oxidation process indicating the minimum energy that reactants must possess for the reaction to proceed, A is the Arrhenius factor, R is the gas constant (8.314 J/(molK)), k is the rate constant and T is the solution temperature. The slope of the straight line was made by plotting ln k versus 1/ T (Fig. 5) and calculated by linear fitting, yielded the apparent activation energy. The activation energy for the homogeneous Fenton oxidation and heterogeneous Fenton oxidation processes was 38.18 kJ/mol and 19.22 kJ/mol respectively. 3.3.3. Analysis of thermodynamic parameters The thermodynamic parameters Gibbs free energy (ΔG), change in enthalpy (ΔH), and change in entropy (ΔS) were calculated for the homogeneous and heterogeneous Fenton oxidation processes using classical Van't Hoff equation. Negative sign of ΔG indicates the spontaneity of the oxidation process. ΔH is used to identify the nature of reaction, either exothermic or endothermic. The positive value of ΔS indicates the increased randomness of reaction i.e. tendency to enter into solution phase. The enthalpy and entropy 0.0031 0.0032 0.0033 0.0034 Fig. 5. Determination Ea values for homogeneous and heterogeneous Fenton Oxidation of textile wastewater. changes are related to the equilibrium constant by the following expression [18]: lnKe ¼ is the COD of wastewater at time t = 0 is the COD of wastewater at any time “t” is time in minutes is the rate constant, L/mg.min 0.003 1/T (1/K) Where, [A]0 [A]t t k without MAC 0 0.0029 ΔS ΔH − R RT ð8Þ where, Ke is the reaction equilibrium constant, the ΔH and ΔS values were obtained from the slope and intercept of the plot (lnKe versus 1/T). The values are presented in Table 3. The positive value of ΔH indicates that the oxidation process is an endothermic process and the positive value of ΔS indicates the random orientation of the converted Table 2 Characteristics of raw textile wastewater and Fenton oxidized textile wastewater. S. No Parameters 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12 12. 13. 14. 15. Raw After integrated homogeneous and wastewater heterogeneous Fenton oxidation pH 8.31 Chemical oxygen 564 demand Biochemical oxygen 120 demand Total organic carbon 144 Oxidation reduction + 525 potential (mV) Total solids 7508 Total dissolved solids 7148 Total suspended solids 360 Chloride 1749 Sulfate 1605 Ammonia 8.4 TKN 16.2 Total hardness 400 60 Ca 2+ Volatile dissolved solids 1.064 Surfactant 9.71 4.5 51 20 25 + 680 7028 6752 276 1582 1281 Nil 6.0 260 46 0.347 1.53 All the values except pH an ORP are expressed in mg/L. Table 3 Thermodynamic properties of Homogeneous Fenton oxidation (without MAC800) and Heterogeneous Fenton Oxidation (with MAC800) of textile wastewater. S. Temperature (K) No Homogeneous Fenton oxidation ΔG(kJ/ mol) 1 2 3 4 1.1960 0.4173 − 0.3617 − 1.1407 ΔH(kJ/ mol) ΔS(J/ molK) 24.80 77.934 Heterogeneous Fenton Oxidation ΔG(kJ/ mol) − 2.071 − 2.819 − 3.567 − 4.315 ΔH(kJ/ mol) ΔS(J/ molK) 20.593 74.793 442 S. Karthikeyan et al. / Desalination 281 (2011) 438–445 products at the solid–solution interface. The linear equation is lnKe = −2983.3 (1/T) + 9.373 and the correlation coefficient 0.9962 for homogenous Fenton oxidation process and lnKe = −2477.9 (1/T) + 8.996 with correlation coefficient 0.9916 for heterogeneous Fenton oxidation process. The free energy of oxidation reaction (ΔG) was related with ΔH and ΔS by the following mathematical expression [16–18], ΔG ¼ ΔH TΔS ð9Þ The ΔH and ΔS values were 24.80 kJ/mol and 77.93 J/molK for homogeneous Fenton oxidation and 20.59 kJ/mol and 74.79 J/molK for heterogeneous Fenton oxidation of textile wastewater. The values suggest that MAC800 has lowered the enthalpy of the reaction in heterogeneous Fenton reaction. The entropy of the Fenton reaction was reduced substantially from 77.93 J/molK to 74.79 J/molK in the presence of MAC800, which could be attributed to the surface condensation of molecules; consequently the enthalpy of the reaction was also decreased. The lowering of the enthalpy of reaction could be due to the presence of active energy sites in MAC800. The presence of active energy sites could be confirmed from the evidence of free electrons in MAC800, which arises from the cleavage of C\C bonds in the network structure of carbon matrix. It was indicated in this manuscript under the Introduction section that the hydroxyl radical formation and sustainability of the reaction could be possible only with the condition that the electron was abstracted from hydroxides generated as a result of Fenton reaction (Eq. (1)). The energy band model for MAC presents that valence band and conduction band are separated by the energy gap 1.55 eV (determined using UV reflectance spectroscopy). The valence band is the source for abstracting an electron from hydroxide ion and conduction band is the source for generating back Fe2+ from Fe3+ so that the sustainability of the Fenton reaction is established. Thus, the activation energy required for heterogeneous Fenton oxidation of organics in the wastewater is lesser than homogeneous Fenton oxidation. The lowering of activation energy in heterogeneous Fenton oxidation of organics clearly demonstrates the part played by MAC800 as a heterogeneous catalyst rather than an adsorbent in the treatment of textile wastewater. Thus, MAC800 has been considered as the heterogeneous catalyst in the present investigation. 3.4. Effect of dosage of MAC800 Fig. 6 illustrates the percentage removal of COD as a function of mass of MAC800. The percentage removal of COD increased with the mass of MAC800 (2 g/L to 25 g/L) with the minimum COD reduction of 80% (for MAC800 2 g/L) and with maximum COD reduction of 90% (for MAC800 10 g/L) in 4 h and thereafter it remained constant. The substantial increase in COD removal could be attributed to the increase in the number of reaction sites for accommodating hydroxyl radicals and pollutants in wastewater. The active sites of the MAC800 were accessible to adsorption of organics according to the energy possessed by them. The pH of the treated wastewater was increased from an initial value of 3.5 to 4.9 and ORP value was increased from +525 mV to +680.9 mV. The considerable increase in percentage reduction of COD can be attributed to the presence of MAC800 which has been considered as an extrinsic semi conducting material with Eg 1.55 eV [11]. MAC800 was able to generate hydroxyl radical from oxygen by the electron holes in the valence band of the carbon matrix. Beyond 4 h, no considerable reduction in COD was observed. This could be due to the fact that the OH• produced might be consumed by the organics present in the effluent. Thus, the time of reaction was fixed at 4 h in further studies. The pH of the reaction mixture was increased from 3.5 to 4.8 in 4 h [7]. The increase in pH may be attributed to the formation of hydroxide ion during the Fenton reaction (as shown in Eq. (1)). The ORP value was considerably increased to +630 mV in homogeneous Fenton oxidation and to +680 mV in the presence of MAC from the initial value of +525 mV. The increase in the ORP value is an indication of formation of oxidized products due to the Fenton oxidation of organics in textile wastewater. Moreover, the increase in ORP is a clear evidence for the performance of MAC800 as a heterogeneous catalyst rather than an adsorbent in removing COD from wastewater. The optimum concentration of MAC800, used in further experiments was fixed at 10 g/L. 3.5. Effect of pH Fig. 7. shows that the maximum reduction in COD (65%) was achieved in homogeneous Fenton oxidation of textile wastewater at pH 3.5. On either side of pH 3.5, the percentage reduction in COD was less compared to the condition at pH 3.5. The data obtained showed that the decolorization rates were significantly greater in weakly acidic solutions (pH 5 and 6) compared with weakly alkaline ones (pH 7 and 8). These findings are consistent with the results reported by other researchers [19–21]. The efficiency of Fenton oxidation was decreased at pH higher than 6 which could be due to the precipitation of Fe 3+ by the available hydroxyl ions. Ferric hydroxide formed could decompose H2O2 into oxygen and water, and consequently the oxidation rate was decreased owing to the presence of low concentration of hydroxyl radicals [22]. Moreover at pH greater than 5 the complexation of Fe2+ as [Fe (II) (H2O) 6]2+ was favored which reacted more slowly with H2O2 than with [Fe (II) (OH) (H2O) 5]2+ and thus producing less amount of hydroxyl radicals. The effect was reversed at low pH, as the surplus hydrogen ions favor the backward reaction (as shown in Eq. (10)) leading to an increase in Fe3+ concentration. 3þ Fe þ H2 O2 →Fe 95 85 % COD Removal % COD Removal 90 80 75 70 65 60 0 5 10 15 20 25 Mass of MAC, (g/L) Fig. 6. Percentage removal of COD as a function of mass of MAC800. OOH 2þ þH þ ð10Þ 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0 With MAC Without MAC 2.5 3.0 3.5 4.0 4.5 pH Fig. 7. Percentage removal of COD as a function of pH. 443 S. Karthikeyan et al. / Desalination 281 (2011) 438–445 Therefore, the conversion of Fe 3+ to Fe 2+ was inhibited, and thus the concentration of Fe 2+ available for the generation of hydroxyl radical was decreased substantially. The findings of this study are consistent with other results reported in the literature [23–25]. organics in textile effluent. The above sequences were in accordance with the following equations: H2 O2 →HO• þ HO• ð11Þ 3.6. Effect of [H2O2]:[FeSO4.7H2O] Fe2þ þ HO• →Fe3þ þ OH− ð12Þ The pH was fixed at pH 3.5 and the time of reaction was fixed at 4 h in the batch reactor. The hydrogen peroxide concentration was varied keeping the ferrous sulfate concentration constant i.e. in the ratio of 1:1, 2:1, 3:1 and 4:1 respectively. In another experiment the H2O2 concentration was kept constant and the concentration of FeSO4.7H2O was varied in the ratio of 1:2, 1:3 and 1:4 respectively. Figs. 8 and 9 show that the maximum COD reduction was observed at the ratio of 2:1 for H2O2:FeSO4.7H2O in both homogeneous and heterogeneous Fenton Oxidation of textile wastewater. About 65% reduction in COD could be achieved in the absence of MAC800 (Homogeneous) and 91% reduction in COD in the presence of MAC800 (heterogeneous). The concentration of HO• radical increased with increase in H2O2 dosage which leads to increased rate of oxidation of organic compounds (COD). The retarded rate of decomposition of COD at higher ratios was due to the fact that the hydroxyl radicals generated are converted into hydroxyl ions which in turn precipitate Fe 3+ ions. As a result of this, the concentration of Fe 2+ was depleted in the system leading to a decrease in the rate of removal of dissolved h ð13Þ i Fe3þ ½OH− Š→Ksp FeðOHÞ3 In other words, the deactivation of Fe 2+ takes place at higher dosages of H2O2, thus giving a brownish yellow precipitate at dosages above 4 mM/L. The use of high concentration of Fe 2+ (for a given hydrogen peroxide concentration) accelerated the rate of decomposition of the hydrogen peroxide generating per hydroxyl radicals [HO2•] which is considered to be the rate limiting step [26], while lower doses of Fe 2+ favored the reaction to generate OH• which is more reactive than the HO2• radicals [27]. Beyond the dosage, 6 mM/L inactivation of the Fe 2+ occurred on account of instantaneous formation of hydroxyl ion in accordance with the following reaction: OH• þ Fe2þ →OH− þ Fe3þ ð14Þ This takes place at a faster rate (3.2 × 10 8 M − 1 s − 1) than the formation of hydroxyl radical from Fe 2+ and hydrogen peroxide (70 M − 1 s − 1). The reaction (14) favors the formation of Fe (OH) 3 nuclei on exceeding the solubility product of ferric hydroxide (Ksp). 3.7. Integrated homogenous and heterogeneous Fenton oxidations of dissolved organics in textile wastewater 100 % COD Removal 90 80 70 60 50 With MAC Without MAC 40 30 20 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Concentration of FeSO4 , mM/L Fig. 8. Percentage removal of COD as a function of FeSO4.7H2O concentration. The characteristics of treated wastewater are shown in Table 2. The Integrated homogenous and heterogeneous Fenton oxidations resulted in COD reduction by 91%, consequently, a considerable amount of reduction in TOC (83%) and in BOD5 (83%) was observed under heterogeneous Fenton oxidation. The pH value of the wastewater sample after heterogeneous Fenton oxidation was 4.6 while the ORP value increased to +681 mV from an initial value of +523 mV. The results clearly showed that the concentration of products formed due to the oxidation process was higher during the integrated Fenton oxidation than the non-oxidized compounds in the raw wastewater. The BOD/COD ratio, an indication of biodegradable index of wastewater was altered significantly during the progress of integrated Fenton oxidation of textile wastewater as shown in Fig. 10. This also supports the fact that oxidized products (having higher biodegradability) are formed indicated by high ORP during Fenton oxidation. 0.7 95 600 90 85 500 BOD/COD 75 70 65 60 55 With MAC Without MAC 50 45 0.4 0.3 COD,(mg/L) 0.5 80 % COD Removal COD BOD/COD 0.6 400 300 0.2 200 0.1 100 0.0 0 40 35 0 0 2 4 6 8 10 12 14 16 Concentration of H2O2 , mM/L Fig. 9. Percentage removal of COD as a function of H2O2 concentration. 50 100 150 200 250 300 350 400 Time, (min) Fig. 10. Variation of COD and BOD/COD as a function of time in the Integrated Fenton oxidation of textile wastewater. 444 S. Karthikeyan et al. / Desalination 281 (2011) 438–445 3.8. Effect of space time application The space time (τ) application of the influent was determined on the basis of void volume in the bed [28] and the applied flow rate. The space time application is expressed as τ¼ εAH Q ð15Þ In the above expression (Eq. (15)) Q is the applied flow rate in cm 3/min, A is the cross-sectional area of the reactor in cm 2, H is the height of the carbon bed in cm and ε is the porosity (0.5) of the carbon bed and the reactor was run at six space time applications. Fig. 11 shows the effect of space time application on the removal of COD and BOD5. The COD in the treated textile wastewater was decreased substantially to 42 mg/L from the influent value of 564 mg/L at τ of 90.43 min for a carbon bed height of 16 cm. The efficiency of COD removal was poor with the other two carbon bed heights of 8.5 and 11.5 cm. Therefore, the optimized height of the carbon bed considered for the Heterogeneous Fenton Oxidation in continuous mode configuration was 16 cm. For each of the bed height, larger space time applications established a greater COD removal than those with small space time application [29]. It is known that a small space time application was associated with an increased superficial velocity of wastewater which resulted in poor contact time with the carbon matrix. The weaker sorption of the pollutants onto the carbon matrix for oxidation at lower space time applications might be the reason for the poor removal of organics from wastewater. 3.9.2. FT-IR studies FT-IR spectra of raw textile wastewater, after homogeneous and heterogeneous Fenton oxidations are presented in Fig. 13. A broad peak at 3336.24 cm− 1 is attributed to N–H and O–H stretching vibrations. An intense strong peak at 1423.21 cm− 1 is due to the presence of C–N stretching. The intensity of peaks at 1639.43 cm− 1, 1638.39 cm− 1 and 1638.77 cm− 1 in the initial, homogeneous and heterogeneous Fenton oxidized wastewater samples respectively whose intensity was decreased due to the partial oxidation of the compounds [31]. Thus, it can be concluded that the integrated homogeneous and heterogeneous Fenton oxidation systems substantially eliminated the chromophoretic compounds and aromatic nuclei in the textile wastewater. 3.9.3. Cyclic voltammetric studies Fig. 14a portrays the cyclic voltagram (CV) of the raw textile wastewater. A cathodic peak at a potential of −0.6 V and another anodic peak at a potential of −0.4 V was observed in the reverse scan of the raw 3.9. Instrumental analysis 3.9.1. UV–Visible spectrophotometric studies The UV–Visible spectrum of the raw wastewater shows the characteristic peaks at wavelengths 240, 270, 353 and 670 nm. The peaks at 240 nm and 270 nm may be attributed to aromatic compounds present in the wastewater. The peak corresponds to 670 nm may be attributed to the chromophoretic compounds present in the wastewater. The UV–Vis spectrum recorded for the textile wastewater sample after homogeneous Fenton oxidation shows that the peaks at λmax 240, 270, 353 and 670 nm remained with substantial reduction in the intensity [30]. The UV–Vis spectrum of the textile wastewater sample after heterogeneous Fenton oxidation showed that the peaks at λmax 270, 353 and 670 nm were eliminated. This implied that the aromatic compounds and chromophoretic groups were destabilized under the conditions of heterocatalytic oxidation. The extent of decolorization at different residence time of oxidation is presented in Fig. 12. Fig. 11. COD remaining as a function of space time application. Fig. 12. UV–Visible Spectra of the textile wastewater before and after integrated Fenton oxidation of textile wastewater at different time interval. Fig. 13. IR spectra of wastewater samples a) before Fenton oxidation, b) after Homogeneous Fenton oxidation and c) after Heterogeneous Fenton Oxidation. S. Karthikeyan et al. / Desalination 281 (2011) 438–445 Fig. 14. Cyclic Voltagram (CV) of textile wastewater samples a) before Fenton oxidation b) after Homogeneous Fenton oxidation and c) after Heterogeneous Fenton Oxidation. textile wastewater and they were eliminated in the homogeneous and heterogeneous Fenton oxidized wastewater samples. It was observed that some of the refractory organic compounds present in the wastewater were oxidized under integrated Fenton oxidation. The voltagram of treated textile wastewater samples under homogeneous and heterogeneous Fenton oxidations showed a residual concentration of organic compounds as presented in Fig. 14b and c. This may be correlated with the residual COD in the Fenton oxidized textile wastewater. These residual compounds appear to resist Fenton oxidation process. 4. Conclusions The optimum time and pH required for the treatment of the textile wastewater were found to be 4 h and 3.5 respectively. There was a substantial removal of pollution parameters from the textile wastewater at low dosages of Fenton's reagent in the presence of MAC800 without sludge production. The thermodynamic properties such as free energy, enthalpy and entropy of the reaction were determined. The positive value of ΔH, 24.80 kJ/mol for homogeneous Fenton oxidation and 20.593 kJ/mol for heterogeneous Fenton oxidation indicated that the process was endothermic. The positive value of ΔS, 77.934 J/molK for homogeneous Fenton oxidation and 74.793 J/molK for heterogeneous Fenton oxidation showed the increased randomness of the oxidized products. The ΔG values were lesser for heterogeneous Fenton oxidation than for homogeneous Fenton oxidation process. The UV–Visible, FTIR and CV spectra confirmed the oxidation of dissolved organic compounds in the textile wastewater. 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